Near field transducer using dielectric waveguide core with fine ridge feature

ABSTRACT

An apparatus for energy assisted magnetic recording of a storage disk includes a plurality of dielectric waveguide cores disposed near an air bearing surface of a magnetic recording device. Each waveguide core has a fine ridge feature on a first surface of the waveguide core and configured to receive incident light energy from an energy source. A near field transducer (NFT) is formed at the air bearing surface for focusing light energy received from the waveguide core and transmitting the focused light energy onto the storage disk surface to generate a heating spot. The NFT includes at least one plasmonic metal element disposed above the fine ridge features of the waveguide cores to form an interface for delivering propagating surface plasmon polaritons (PSPPs) to the air bearing surface. Each fine ridge feature is configured with a width approximately equivalent to a width of the heating spot.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional ApplicationSerial No. 62/010,057 filed on Jun. 10, 2014, which is expresslyincorporated by reference herein in its entirety.

BACKGROUND

High density storage disks are configured with layers of materials thatprovide the required data stability for storage. The magnetic propertiesof the media may be softened when writing to the disk to assist changingthe bit state. Energy Assisted Magnetic Recording (EAMR) device or HeatAssisted Magnetic Recording (HAMR) technology provides heat that isfocused on a nano-sized bit region when writing onto a magnetic storagedisk, which achieves the magnetic softening. A light waveguide directslight from a laser diode to a near field transducer (NFT). The NFTfocuses the optical energy to a small spot on the target recording areawhich heats the magnetic storage disk during a write operation.Inefficiencies in the NFT can have a negative impact on the power budgetof the laser diode and the EAMR/HAMR system lifetime. Higher NFTefficiency allows for lower laser power demand, relieving EAMR/HAMRsystem requirement on the total optical power from the laser source, andresults in less power for parasitic heating of the EAMR/HAMR headresulting for improved reliability.

In the NFT, plasmonic metal can be used to interface with an energizeddielectric waveguide for propagating surface plasmon polaritons (SPPs),which carry out the nano-focusing function beyond the light'sdiffraction limit. High quality plasmonic metals rely on high densityfree electrons which have weak mechanical robustness, and aresusceptible to damages caused by thermal or mechanical stresses presentin an EAMR head. Under these stresses, the service lifetime of theEAMR/HAMR device is limited to NFT failure occurring at the plasmonicmetal part having fine (nano-sized) features, such as at a ridge or apin.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present invention will now be presented in thedetailed description by way of example, and not by way of limitation,with reference to the accompanying drawings, wherein:

FIG. 1 shows a diagram of an exemplary hard disk drive;

FIG. 2 shows a diagram of an exemplary embodiment of a dielectricwaveguide core pair, each waveguide core having a fine ridge feature toform an interface with a nearby near field transducer (NFT) element forheat assisted magnetic recording on a storage disk;

FIG. 3 shows a diagram of an alternative exemplary embodiment with afine ridge feature on the dielectric waveguide core being offset fromthe centerline of the waveguide core; and

FIG. 4 shows a diagram of an alternative exemplary embodiment for inwhich the waveguide core has two fine ridge features.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various exemplary embodimentsand is not intended to represent the only embodiments that may bepracticed. The detailed description includes specific details for thepurpose of providing a thorough understanding of the embodiments.However, it will be apparent to those skilled in the art that theembodiments may be practiced without these specific details. In someinstances, well-known structures and components are shown in blockdiagram form in order to avoid obscuring the concepts of theembodiments. Acronyms and other descriptive terminology may be usedmerely for convenience and clarity and are not intended to limit thescope of the embodiments.

The various exemplary embodiments illustrated in the drawings may not bedrawn to scale. Rather, the dimensions of the various features may beexpanded or reduced for clarity. In addition, some of the drawings maybe simplified for clarity. Thus, the drawings may not depict all of thecomponents of a given apparatus.

Various embodiments will be described herein with reference to drawingsthat are schematic illustrations of idealized configurations. As such,variations from the shapes of the illustrations as a result ofmanufacturing techniques and/or tolerances, for example, are to beexpected. Thus, the various embodiments presented throughout thisdisclosure should not be construed as limited to the particular shapesof elements illustrated and described herein but are to includedeviations in shapes that result, for example, from manufacturing. Byway of example, an element illustrated or described as having rounded orcurved features at its edges may instead have straight edges. Thus, theelements illustrated in the drawings are schematic in nature and theirshapes are not intended to illustrate the precise shape of an elementand are not intended to limit the scope of the described embodiments.

The word “exemplary” is used herein to mean serving as an example,instance, or illustration. Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. Likewise, the term “embodiment” ofan apparatus or method does not require that all embodiments include thedescribed components, structure, features, functionality, processes,advantages, benefits, or modes of operation.

As used herein, the term “about” followed by a numeric value meanswithin engineering tolerance of the provided value.

In the following detailed description, various aspects of the presentinvention will be presented in the context of an interface between awaveguide and a near field transducer used for heat assisted magneticrecording on a magnetic storage disk.

FIG. 1 shows a hard disk drive 111 including a disk drive base 114, atleast one rotatable storage disk 113 (e.g., such as a magnetic disk,magneto-optical disk), and a spindle motor 116 attached to the base 114for rotating the disk 113. The spindle motor 116 typically includes arotating hub on which one or more disks 113 may be mounted and clamped,a magnet attached to the hub, and a stator. At least one suspension arm108 supports at least one head gimbal assembly (HGA) 112 that holds aslider with a magnetic head assembly of writer and reader heads. A rampassembly 100 is affixed to the base 114, and provides a surface for tipof the suspension arm 108 to rest when the HGA 112 is parked (i.e., whenthe writer and reader heads are idle). During a recording operation ofthe disk drive 111, the suspension arm 108 rotates at the pivot 117,disengaging from the ramp assembly 100, and moves the position of theHGA 112 to a desired information track on the rotating storage disk 113.During recording, the slider is suspended by the HGA 112 with an airbearing surface of the slider that faces the rotating disk 113, allowingthe writer head to magnetically alter the state of the storage bit. Forheat assisted magnetic recording, a near field transducer (NFT) on theair bearing surface may couple light energy from a waveguide to producea heating spot on the rotating disk 113 for magnetically softening thebit space.

FIG. 2 shows a diagram of an exemplary embodiment of an NFT 200 andwaveguide core arrangement. The NFT 200 may be formed by a plasmonicmetal element 205, which may be aligned with the air bearing surface(ABS) 210 of a slider. The surface of the recording medium 113 (e.g., arotating storage disk) is exposed to the ABS 210. As the slider fliesover the recording media 113, a cushion of air is maintained between theslider and the disk. In this embodiment, two dielectric waveguide (WG)cores 211, 212 are arranged to each carry light energy toward the ABS210. The light energy may be generated by a common laser diode source(not shown) that may be split in half by a splitter (not shown). Thedielectric waveguide cores 211, 212 may be of equal length to ensurethat the combined energy wave at the ABS 210 is in substantial phasealignment for constructive interference and maximum energy emission tothe surface of the recording medium 113. Alternatively, dielectricwaveguide cores 211, 212 may be of unequal length such that the incidentenergy waves may have a particular phase difference that optimizesconstructive interference and maximum energy magnitude at the ABS 210.The two waveguide cores 211, 212 are substantially linear and convergeat a junction near the ABS 201 at an interior angle between 0 and 180degrees, (e.g., approximately 90 degrees as shown in FIG. 2).

As shown in FIG. 2, each waveguide core 211, 212 has a fine ridgefeature 201, 202. along the surface of the waveguide core 211, 212facing the plasmonic metal element 205. For illustrative purpose, theplasmonic metal element 205 is depicted as transparent to reveal thefine ridge features 201, 202 below. The optical energy from the fineridge feature 201, 202 of the dielectric waveguide cores 211, 212 inproximity with the plasmonic metal element 205 energizes propagatingsurface plasmon polaritons (PSPPs) along the plasmonic metal surfacetoward the ABS. The fine ridge feature 201, 202 of the waveguide core211, 212 may be configured to have a width substantially less than thewidth of the waveguide core 211, 212. For example, the ridge feature201, 202 may be about 10-70 nm wide compared to waveguide core 211, 212having a width of about 300-500 nm. Also, the height of the fine ridgefeature may be about 10-70 nm for example. Because the fine ridgefeature 201, 202 is formed on the dielectric waveguide core 211, 212,which is made of a material more robust than plasmonic metal, thereliability and durability of the NFT 200 may be improved.

The dielectric material of the waveguide core 211, 212 and the fineridge feature 201, 202 may be Ta₂O₅ for example. The material of theplasmonic metal element 205 may be a gold alloy, for example. Otherexamples of plasmonic metals that may be used to form the metal barelement include silver or copper alloys. A gap (e.g., 20 nm) may existbetween the plasmonic metal element 205 and the fine ridge feature 201,202 of the dielectric waveguide core 211, 212. Alternatively, the gapmay be omitted, and the plasmonic metal element 205 may directly contactthe fine ridge feature 201, 202, at least for a portion of the interfacebetween the fine ridge feature 201, 202 and the plasmonic metal element205. The two dielectric waveguide cores 211, 212 and the entireplasmonic metal element 205 may be encapsulated by a silicon oxidematerial.

The fine ridge features 201, 202 may be configured as shown in FIG. 2,converging at a junction above the junction of the dielectric waveguidecores 211, 212. The junction of fine ridge feature 201, 202 may occur ona common plane. The junction of fine ridge feature 201, 202 may beformed near at the ABS, with an extension 203 arranged perpendicular tothe ABS 210 as shown in FIG. 2. Alternatively, the ABS 210 may be formeddirectly at the junction of the fine ridge features 201, 202, thuseliminating the extension 203. The precision of the delivered heatingspot size at the recording disk 113 surface may be based on the exposedwidth of the fine ridge feature extension 203, or exposed junction 201,202 at the ABS 210. There may be a direct correspondence between thewidth dimension of the exposed fine ridge feature 203 and the heatingspot size. Alternatively, other factors may affect the heating spotsize, such as the length of the waveguide cores 211, 212, which mayaffect the constructive interference of the incident light energy at theABS 210. The target size of the heating spot on the recording disk 113is dependent on the track size as the slider flies over the track, whichmay be between 10-70 nm wide for example. The size of the heating spotalso depends on the distance between the ABS 210 and the disk 113. Thefocus of the heating spot may be optimized by minimizing the gapdistance.

The thickness of the plasmonic metal element 205 is not a significantfactor in achieving the precise nano-sized heating spot, and thereforethe thickness may be configured according to providing adequate heattransfer for controlling the peak temperature in the NFT 200. Also,having the fine ridge feature 201, 202 of the waveguide core 211, 212serving as the energy focusing feature for PSPP activity, may allow theplasmonic metal element 205 to have relaxed shape and size requirements(i.e., length and width dimensions) for which a fine feature is notnecessary. The plasmonic metal element 205 configuration is determinedby a shape and size necessary to interact with the fine ridge features201, 202 (i.e., a footprint that covers the fine ridge features 201,202). As an example, the size of a semi-circle shaped plasmonic metalelement 205 may be 1000 nm in diameter and greater than 100 nm inthickness. While shown in FIG. 2 as a semi-circle shape, the plasmonicmetal element 205 may be configured in shapes other than a semi-circle,such as a rectangular or polygonal block.

The NFT 200 configuration as shown in FIG. 2 having two fine ridgefeatures 201, 202 serving as PSPP elements may provide approximatelytwice as much electrical field magnitude compared with a NFTconfiguration having only a single fine ridge feature with plasmonicmetal element at the ABS 210, driven by a common total input power inthe waveguide system. The constructive interference produced by the twoPSPP elements allows improved efficiency of energy delivery from thelaser diode source, which translates to longer service life of theEAMR/HAMR device. To optimize the efficiency of the two PSPP elementconfiguration, each PSPP element 201, 202 is configured with a length Lwhich is an integer multiple of coupling length Lc from dielectricwaveguide core to the PSPP element 201, 202 (e.g., for Lc of 1200 nm,the length of the PSPP element should be about a(1200 nm) where a is aninteger value). With the PSPP element having a length L approximatelyequivalent to aLc ensures that the maximum energy transfer propagatesfrom the PSPP element 201, 202 at the ABS. If the length of the PSPPelement deviates from aLc, some of the energy wave may be lost to thedielectric waveguide core 211, 212.

The NFT 200 does not have to be limited to two interfering PSPP elementsas shown in FIG. 2. In an alternative embodiment, N (a positive integer)PSPP elements interfere at the ABS 210, which may provide approximatelyN times increase of the electrical field magnitude, driven by a commontotal input power in the waveguide system. The value of N can increasebeyond 2 or 3, until other parasitic interferences within the threedimensional layout the EAMR head becomes a limiting factor. For N>3, thePSPP elements may be arranged in a three dimensional configuration. Theplasmonic metal element 205 would also conform to a corresponding threedimensional configuration to provide the PSPP interface with the fineridge features for the NFT 200.

FIG. 3 shows an alternative exemplary embodiment in which the fine ridgefeature 202 of the dielectric waveguide core is offset from thecenterline of the waveguide core. Hence, the fine ridge feature 202 maybe fabricated with a deviation from the centerline of the waveguide core212 and still interface with the plasmonic metal element 205 for afunctional PSPP interface. In a similar manner, the fine ridge feature201 may be formed offset from the centerline of the waveguide core 211.In this alternative embodiment, the fine ridge features 201, 202converge at a junction point near the ABS 210 as in FIG. 2, either withan extension 203, or at the ABS 210 without the extension 203.

The number of PSPP elements may vary with respect to the number of fineridge features on dielectric waveguide cores. FIG. 4 illustrates anexample where the waveguide core 202 has two fine ridge features 202disposed side-by-side below the plasmonic metal element 205.

The various aspects of this disclosure are provided to enable one ofordinary skill in the art to practice the present invention. Variousmodifications to exemplary embodiments presented throughout thisdisclosure will be readily apparent to those skilled in the art, and theconcepts disclosed herein may be extended to other devices. Thus, theclaims are not intended to be limited to the various aspects of thisdisclosure, but are to be accorded the full scope consistent with thelanguage of the claims. All structural and functional equivalents to thevarious components of the exemplary embodiments described throughoutthis disclosure that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the claims. Moreover, nothingdisclosed herein is intended to be dedicated to the public regardless ofwhether such disclosure is explicitly recited in the claims. No claimelement is to be construed under the provisions of 35 U.S.C. §112(f)unless the element is expressly recited using the phrase “means for” or,in the case of a method claim, the element is recited using the phrase“step for.”

What is claimed is:
 1. An apparatus for energy assisted magneticrecording of a storage disk, comprising: a plurality of dielectricwaveguide cores disposed near an air bearing surface of a magneticrecording device, each waveguide core comprising a fine ridge feature ona first surface of the waveguide core and configured to receive incidentlight energy from an energy source; and a near field transducer formedat the air bearing surface for focusing light energy received from thewaveguide core and transmitting the focused light energy onto thestorage disk surface to generate a heating spot, comprising: at leastone plasmonic metal element disposed above the fine ridge features ofthe waveguide cores to form an interface for delivering propagatingsurface plasmon polaritons (PSPPs) to the air bearing surface; whereineach fine ridge feature is configured with a width approximatelyequivalent to a width of the heating spot.
 2. The apparatus of claim 1,wherein the interface comprises a gap between at least one plasmonicmetal element and the fine ridge features of the waveguide cores.
 3. Theapparatus of claim 1, wherein each of the waveguide cores andcorresponding fine ridge features is substantially linear and comprisesa first end and a second end, and the first ends of all fine ridgefeatures are connected together at a junction point near the air bearingsurface with at least a portion of the connected fine ridge features isexposed on the air bearing surface.
 4. The apparatus of claim 3, whereinthe junction point of the first ends further comprises a fine ridgefeature extension configured perpendicular to the air bearing surfaceand exposed on the air bearing surface.
 5. The apparatus of claim 1,wherein the at least one plasmonic metal element is configured as asingle element arranged to cover all of the fine ridge elements, theplasmonic metal element having a variable thickness and a substantiallyflat surface facing the fine ridge elements.
 6. The apparatus of claim4, wherein the at least one plasmonic metal element is configured as asemi-circular disk having a straight edge aligned with the air bearingsurface.
 7. The apparatus of claim 1, wherein the fine ridge feature isoffset from the centerline of the waveguide core.
 8. The apparatus ofclaim 1, wherein at least one of the waveguide cores comprises at leasttwo fine ridge features on a common waveguide core surface.
 9. Theapparatus of claim 1, wherein the width of the heating spot isapproximately equivalent to a width of physical space on the storagedisk reserved for a single data bit.
 10. The apparatus of claim 1,wherein the waveguide cores and the plasmonic metal element areencapsulated by a silicon oxide material.
 11. The apparatus of claim 1,wherein at least three waveguide cores and corresponding fine ridgefeatures are arranged in a three dimensional configuration to interfacewith the at least one plasmonic metal element at the air bearingsurface.
 12. A magnetic hard disk drive, comprising: a rotatablemagnetic storage disk; a laser diode; a plurality of dielectricwaveguide cores disposed near an air bearing surface of a magneticrecording device, each waveguide core comprising a fine ridge feature ona first surface of the waveguide core and configured to receive incidentlight energy from an energy source; and a near field transducer formedat the air bearing surface for focusing light energy received from thewaveguide core and transmitting the focused light energy onto thestorage disk surface to generate a heating spot, comprising: at leastone plasmonic metal element disposed above the fine ridge features ofthe waveguide cores to form an interface for delivering propagatingsurface plasmon polaritons (PSPPs) to the air bearing surface; whereineach fine ridge feature is configured with a width approximatelyequivalent to a width of the heating spot.